440 j. k. mills — locks ol the sierra nevada of california. 



Alteration Products. 



The quartzitic Alteration. — The details of metamorphism belong to 

 lithology, but the quartzitic alteration is so general and on so large a 

 scale in the Sierra that it becomes an essential and characteristic feature 

 of the geology of the range. As before shown, there is quartzite after 

 granite near the Sierra, if not within the range, and on a large scale after 

 slate-, both pre-Mesozoic and Mesozoic, and after greenstone and serpen- 

 tines, and less certainly perhaps but to all appearance, to the naked eye, 

 after limestone. In places the quartzite passes into pure white quartz. 

 Quartz is found in lenticular masses and veinlets isolated from any 

 fissure, in the quartzites and in the slates, and in fact in all the rocks, 

 and such deposits of quartz are especially numerous in the pre-Mesozoic 

 slates; and finally, quartz occupies much the greater part of the space 

 between the walls of fissures throughout the Sierra. 



Pyritom Character of tin Rocks. — Another characteristic which is so 

 prevalent that it cannot he omitted from a geological account of the 

 range is the abundance of pyrite in the slates. From the outcrops alone 

 no adequate idea of the proportion of pyrites could he obtained, hut the 

 more recent erosion and the tunnels and other mining excavations show 

 a widespread distribution of pyrite throughout the slates. On account 

 of the presence of pyrite, the slates weather to yellow and red colors at 

 their outcrops : indeed, the color of the debris resting on the outcrops can 

 he taken as an indication of the age of tin/ surfac< — the debris on surfaces 

 formed by more recent erosion is of gray color, while at surfaces as old as 

 early Quaternary, or. more decidedly, as old as late Tertiary, the debris 

 is of red and yellow colors. Of the pyrite in the green-tones or diabases 

 I cannot speak with confidence; near fissures I have seen greenstone 

 very pyritous. From the results of microscopic examination before 

 quoted, it is probable that the iron in the serpentine is in the form of 

 oxides rather than sulphides. Mas<e< of chromic iron ore are found in 

 the serpentine. 



Fissure* mid mineral Veins. — Quartz, which is so large a product of altera- 

 tion of the rocks of the Sierra, forms the great bulk of the material tilling 

 fissures, and pyrite, winch is so widely distributed in the slates though 

 in far less proportion than quartz, is much more abundant than any 

 other mineral except quartz among the contents of fissures. The fissures 

 are generally, perhaps always, at fault planes; they are effects of uplift- 

 ing forces, and the mass on one side of each fissure is usually, if not 

 always, uplifted farther than on the other. As already stated, the prevail- 

 ing direction of the axes of uplift i< approximately parallel to the strike 

 of the rocks, and consequently this is true of the prevailing direction of 



